One aspect of the invention is an improved process of forming an oxide coating, which I refer to as a "cobalt conversion coating," that is chemically formed by oxidizing the surface of a metal substrate, such as an aluminum or aluminum alloy substrate. The invention enhances the quality of the environment of humankind by contributing to the maintenance of air and water quality.
In general, chemical conversion coatings are formed chemically by causing the surface of the metal to be "converted" into a tightly adherent coating, all or part of which consists of an oxidized form of the substrate metal. Chemical conversion coatings can provide high corrosion resistance as well as strong bonding affinity with paint. The industrial application of paint to metals generally requires the use of a chemical conversion coating, particularly when the performance demands are high.
Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys, particularly the high-copper 2000-series aluminum alloys, such as alloy 2024-T3, corrode much more rapidly than pure aluminum.
In general, there are two types of processes for treating aluminum to form a beneficial conversion coating. The first is by anodic oxidation (anodization) in which the aluminum component is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed through the aluminum component and the chemical bath. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for paint.
The second type of process is by chemically producing a conversion coating, which is commonly referred to as a chemical conversion coating, by subjecting the aluminum component to a chemical solution, such as a chromate solution, but without using an electric current in the process. The chemical solution may be applied by immersion application, by manual application, or by spray application. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for paint. The present invention relates to this second type of process for producing chemical conversion coatings. The chemical solution may be applied by immersion application, by various types of manual application, or by spray application.
One widely used chromate process for forming chemical conversion coatings on aluminum substrates is described in various embodiments in Ostrander et al. U.S. Pat. No. 2,796,370 and Ostrander et al. U.S. Pat. No. 2,796,371, in military process specification MIL-C-5541, and in Boeing Process Specification BAC 5719. These chromate chemical conversion baths contain hexavalent chromium, fluorides, and cyanides, all of which present significant environmental as well as health and safety problems. The constituents of a typical chromate conversion bath, such as ALODINE 1200.RTM., are as follows: CrO.sub.3 -"chromic acid" (hexavalent chromium); NaF--sodium fluoride; KBF.sub.4 --potassium tetrafluoroborate; K.sub.2 ZrF.sub.6 --potassium hexafluorozirconate; K.sub.3 Fe(CN).sub.6 --potassium ferricyanide; and, HNO.sub.3 --nitric acid (for pH control).
Many aluminum and aluminum alloy structural parts, as well as Cd plated, Zn plated, Zn-Ni plated, and steel parts, throughout the aircraft/aerospace industry are currently being treated using this chromate process technology. Chromate conversion films, as formed on aluminum and aluminum alloy substrates, meet a 168 hours corrosion resistance criterion, but they primarily serve as a surface substrate for paint adhesion. Because of their relative thinness and low coating weights (40-150 milligrams/ft.sup.2), chromate conversion coatings do not cause a fatigue life reduction in the aluminum and aluminum alloy structure.
However, environmental regulations in the United States, particularly in California, and in other countries are drastically reducing the allowed levels of hexavalent chromium compounds in effluents and emissions from metal finishing processes. Accordingly, chemical conversion processes employing hexavalent chromium compounds must be replaced. The present invention, which does not employ hexavalent chromium compounds, is intended to replace the previously used chromate process for forming conversion coatings on aluminum and aluminum alloy substrates.